Field of the Invention
The present invention generally relates to a brake apparatus for railway vehicles and, more particularly, to a main piston bushing of a service portion of a brake control valve.
Description of Related Art
China Railways have a standardized pneumatic air brake valve, called 120-1 type, which is similar in overall operation to American Association of Railroads (AAR) pneumatic air brake valves (such as the WABCO ABDX), with a pipe bracket, service valve portion, and emergency valve portion.
The service valve portion of such valves typically includes a body with a central opening that houses a main piston assembly that is housed within the central opening. A main piston bushing is positioned within the central opening of the body of the service valve portion between the central opening and the main piston assembly. The main piston assembly includes a service slide valve and a service graduating valve that is assembled on the service piston. The service slide valve, the service graduating valve, and the main bushing each includes a number of openings and passages extending therethrough that define porting patterns. The service slide valve and the service graduating valve are movable with respect to the piston passageway of the body of the service valve portion, with respect to the main bushing, and with respect to each other. Accordingly, various flow passageways are opened through the service valve portion depending upon the relative position of the service slide valve, the service graduating valve, and the main bushing with respect to each other, and the alignment of the various openings and recesses that define the porting patterns of the service slide valve, the service graduating valve, and the main bushing. The service valve portion will perform various brake valve functions, which involve the flow of pressurized air between various locations within the brake line system, depending upon the relative positioning and alignment. This type of system brake valve control is well known to those having ordinary skill in the art and is utilized in the Chinese 120-1 type valves and in the ABDX and ABDW-type brake control valves, which are manufactured by the WABTEC Corporation.
In the ABDX-type brake control valve, the main piston bushing contains a single spring ramp machined into an interior surface of the bushing. The specific purpose of the spring ramp is to provide a slight delay of the main piston assembly to provide a more aggressive preliminary quick service function. This is done by one end of the main piston assembly's wing spring, which must climb out of the spring ramp in the main bushing, before the service slide valve moves out of its release position.
However, when the main piston assembly moves the service slide valve from its release position to its service position, the only mechanism that stops the service slide valve in this position is a combination of the friction between the service slide valve face and its seat, as well as the rate at which the pressure differential between the auxiliary reservoir and the brake pipe is dissipated across the diaphragm-actuated main piston assembly. Depending upon the specific configuration of the brake valve system, it is critical that the service slide valve be stopped in its service position on a consistent basis. If it does not, the service slide valve could travel further to either its boost position or possible jump position on the initial brake application, depending upon the amount of pressure differential and friction acting upon the main piston assembly.